Help Wanted: Document how to do local development on a plugin that requires an E20/Enterprise license

Context

Extra steps are needed to do local development or test changes on a plugin that requires a Mattermost Enterprise (or E20) license. You will see the following error:

Requires a Mattermost Cloud or Mattermost E20 License.

This message is expected. You can proceed by configuring your server to enable both testing (ServiceSettings.EnableTesting) and Developer mode (ServiceSettings.EnableDeveloper). See https://github.com/mattermost/mattermost-server/issues/17310#issuecomment-823421688 for additional details.

Docs request

Document the process of configuring a Mattermost server to enable testing and development without an Enterprise (or E20) license to unblock others who encounter the same barrier to contribution.

How you can help

• Familiarize yourself with the technical execution details.
• Confirm the error that's returned and update if needed.
• Confirm the details work as documented.
• Collaborate with Mattermost Engineering in the Hacktoberfest channel on the Mattermost Community Server to identify the right place for this information to live on the developer documentation site.
• Create a dev docs PR with your updates.

Start with the linked Mattermost server issue comment and reproduce the reported license error while developing or testing the plugin. Work with Mattermost Engineering to identify the right developer-documentation location, then document and verify the testing and developer-mode configuration so contributors can proceed without an Enterprise license.